The invention relates to a valve device that is intended for a pneumatic brake system of a commercial vehicle and that comprises a valve housing, a pilot piston, which is guided in an axially displaceable manner in the valve housing by way of radial seals, a first working port, a second working port, a pneumatic control input, and an air bleed port.
In a first switched state of the valve device the control input is pressurized, wherein the pilot piston is held in a first end position, sealing a first valve seat, against a spring force, so that the first valve seat seals the second working port against the air bleed port, and further wherein the pilot piston is lifted from a second valve seat, so that the first working port is connected to the second working port.
In a second switched state of the valve device the control input is bled, wherein the pilot piston is lifted from the first valve seat by a spring force, so that the first valve seat connects the second working port to the air bleed port, and further wherein the pilot piston is held in a second end position, sealing the second valve seat, with the spring force, so that the second valve seat seals the first working port against the second working port.
Furthermore, the invention relates to an electrically operable parking brake system that is intended for a pneumatic brake system and that has, as the pilot valve device, a valve device of the above-mentioned type.
The invention also relates to a method for controlling an electrically operable parking brake system, in particular for moving the electrically operable parking brake system from a drive mode into a park mode.
In electrically operable parking brake systems, as described, for example, in DE 10 2008 007 877 B3, a central safety-relevant requirement is that a power failure may not result in a state change of the parking brakes. If the parking brake is in its park mode, then even in the event of a power failure it is necessary to maintain the park mode, in order to prevent in this way the commercial vehicle from unintentionally rolling away in any case. If the parking brake is in a drive mode, then the parking brake may not be abruptly engaged when the power fails, because such a sudden engagement can lead to hazardous situations when the vehicle is in motion.
In order to satisfy these safety-relevant requirements, it is possible to use bistable pilot valves, which can be driven either electrically or pneumatically. The present invention deals with pneumatically driven switching valve devices, which are integrated into an electrically operable parking brake system such that the result is a bistability, which in turn meets the aforementioned safety-relevant requirements.
Electrically operable parking brake systems and, in particular, the aforementioned pneumatic pilot valve devices are configured in such a way that absolute switching reliability is guaranteed. If, therefore, the pressure conditions in the parking brake system are changed in order to cause a changeover of the pilot valve device, then the pilot valve device has to be able to switch reliably—even under any external conditions, in particular, even at low temperature, which can prevent a changeover of the switching valve device especially due to higher friction forces.
There is therefore needed a concept that guarantees the bistability of a pneumatic pilot valve device for an electrically operable parking brake system, so that the result is absolute switching reliability even at low temperatures. In meeting this need, the invention shall offer, in particular, solutions that are characterized by minimum abrasive wear of the system components and, in particular, of the pneumatic pilot valve device.
According to the invention, a valve device is provided that is intended for a pneumatic brake system of a commercial vehicle and that comprises a valve housing, a pilot piston, which is guided in an axially displaceable manner in the valve housing by way of radial seals, a first working port, a second working port, a pneumatic control input, and an air bleed port. In a first switched state of the valve device the control input is pressurized, wherein the pilot piston is held in a first end position, sealing a first valve seat, against a spring force, so that the first valve seat seals the second working port against the air bleed port, and further wherein the pilot piston is lifted from a second valve seat, so that the first working port is connected to the second working port. In a second switched state of the valve device the control input is bled, wherein the pilot piston is lifted from the first valve seat by a spring force, so that the first valve seat connects the second working port to the air bleed port, and further wherein the pilot piston is held in a second end position, sealing the second valve seat, with the spring force, so that the second valve seat seals the first working port against the second working port. The first valve seat is displaceable in the direction of movement of the pilot program.
If the pilot piston is sitting on the first valve seat, then the parking brake system is in a drive mode. In order to attain a park mode from this drive mode, the pilot piston has to be lifted from the valve seat and displaced in the valve housing for this purpose. Such a displacement counteracts, in particular, two effects—that is, both the adhesion of the pilot piston to the valve seat and a high static friction of the radial seals, which can be implemented, for example, as O-rings, guiding the pilot piston in the valve housing. Since the valve seat can be displaced in the direction of motion of the pilot piston in accordance with the invention, it is then possible to separate these two effects. That is, when the pilot piston begins to move initially, only the static friction of the radial seals has to be overcome, because the valve seat follows the pilot piston during the initial movement. Only after the radial seals are barely able to generate a sliding friction that is lower by an order of magnitude does the pilot piston have to be lifted from the valve seat against the adhesion forces.
Working on this basis, it can be provided in a useful way that the first valve seat is loaded with a spring force so that, when the pilot piston leaves its first end position, the first valve seat follows the movement of the pilot piston. This spring force facilitates the initial movement of the pilot piston that is adversely affected by the static friction of the radial seals. If vice versa the pilot valve device is to be moved from its park mode into the drive mode, then the only requirement is that the pilot piston be moved against the spring that is assigned directly to the pilot piston, so that then a comparably low spring force has an effect, if, starting from the park mode, only the high static friction of the radial seals is present. If the pilot piston sits on the first valve seat, then thereafter as the pilot piston continues to move, the spring force, assigned to the first valve seat, also has an effect. However, the resulting higher spring force will not impede the further movement of the pilot piston, because as this pilot piston continues to move, just the sliding friction forces of the radial seals continue to have an effect.
In order to guarantee that the pilot piston will be lifted in a reliable and defined manner from the first valve seat, the invention advantageously provides that the displaceability of the first valve seat is limited such that the first valve seat cannot follow the pilot piston as far as to its second end position. This feature is implemented preferably by a stop that precludes the further movement of the movable valve seat.
The invention is further developed in an especially advantageous way in that an active surface that is part of a valve seat piston bearing the first valve seat and that faces away from the first valve seat is arranged in an air bleed space that is located at the pressure level of the air bleed port in at least the first switched state of the valve device. An active surface of this type is defined as a surface that is defined by more than just bevels or edge regions of the valve seat piston. In particular, the active surface can be formed by the whole or almost the whole surface of the valve seat piston that faces away from the valve seat. Thus, by pressurizing the air bleed port a force is generated that acts in this direction like the spring force, so that in this way the movement of the pilot piston against the static friction of the radial seals is facilitated once again. Since this feature is implemented preferably in such a way that in the drive mode the valve seat piston with its active surface, facing away from the valve seat, is not supported directly on the housing, but indirectly on the housing by way of the spring, it is then possible to adjust the sealing force, acting on the first valve seat, by means of the spring force. This force has to be so large that the first valve seat is sealed with a high degree of certainty, but the force may be set so low that the adhesion of the pilot piston is less than in the case of solutions with a stationary valve seat, and, moreover, owing to the reduced valve seat forces, the abrasive wear of the valve seat, made of an elastomer, is slowed down.
Furthermore, the invention provides an electrically operable parking brake system that is intended for a pneumatic brake system and that has a pilot valve device of the aforementioned type, wherein the first working port can be supplied selectively with compressed air from a compressed air source or can be bled, the second working port can be coupled with a compressed air consumer, the pneumatic control input is coupled at least indirectly with the control input of a relay valve for the parking brake, and the air bleed port is coupled with a pilot and air bleed valve device.
The advantages of such an electrically operable parking brake system have already been discussed above in conjunction with the pilot valve device.
The present invention also relates to a method for moving such an electrically operable parking brake system from a drive mode into a park mode, wherein the air bleed port is pressurized by the pilot and air bleed valve device, while the control input is bled.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.